Modern machinery often uses sensors to measure the movement of various parts in the machinery. These measurements are used to, inter alia, make adjustments in how various parts of the machinery operate, and hence, improve the performance of the machinery. For example, the precise speed with which a shaft having gear teeth thereon rotates may be critical to the operation of an internal combustion engine. In such a case, sensors that are able to precisely measure the speed at which the shaft rotates can be used to provide information to other parts of the engine. Such information can be used, for example, by the part of the engine which controls the timing for the fuel and the spark to improve the performance of the engine.
Hall effect sensors are a class of sensors that are used to, inter alia, detect the movement of ferrous items. As such, they are well suited to uses such as detecting the rotational movement of a shaft in an engine. Specifically, a Hall sensor incorporates a Hall plate that senses the movement of a ferrous item as it passes by the Hall plate. The Hall plate produces a voltage output between two terminals. This voltage output changes as the ferrous item first approaches and then moves away from the Hall plate. When such a Hall plate is mounted adjacent gear teeth on a rotating shaft, the frequency with which the voltage output from the Hall plate increases and decreases is a precise measurement of the rotational speed of the shaft. This voltage output can then be transformed into a different type of electrical signals which may be used by a microprocessor or other circuitry to control various other parts of the engine, for example, the timing of the fuel or spark.
The ability of the Hall sensor to accurately measure movement may significantly affect the performance of machinery such as an engine. Ideally, the output signal of the Hall sensor should immediately begin to increase when a rising edge of a gear tooth approaches the Hall plate and should immediately decline as the gear tooth recedes from the sensor. Additionally, neither manufacturing tolerances nor physical stress on the sensor should affect its operation. In practice, however, timing delay from the Hall sensor circuitry, manufacturing tolerances, physical stress and other factors degrade the performance of the Hall sensor. As the performance of the Hall sensor is degraded, the performance of the machine in which it operates may also be degraded.
In order to avoid some of the practical problems associated with Hall sensors, the prior art has disclosed using a four terminal Hall device such as that shown in FIG. 1. In FIG. 1, the Hall plate 11 incorporates four terminals (2, 4, 6 and 8) connected to two switch pairs 12 and 14 that alternately connect the Hall plate to a voltage source 13 ground 10 or output lines 15, 15'. The switch pairs operate in conjunction with each other so that, for example, when terminal 2 is connected to voltage source 13, terminal 6 is connected to ground 10 terminal 8 is connected to output line 15 and terminal 4 is connected to output line 15'. As a result, a voltage output is developed on output lines 15, 15'. Alternatively, when terminal 4 is connected to ground 10, terminal 8 is connected to voltage source 13 then the voltage output lines are connected to terminals 2 and 6.
The switch pairs 12 and 14 continually switch back and forth in response to a clock signal (not shown). In this way, the source of the output voltage continually switches from terminal pair 4 and 8 to terminal pair 2 and 6. By switching the output voltages between the two terminal pairs, the effects of practical problems such as manufacturing tolerances and physical stress are minimized. This is because the measured output voltage signal (V.sub.out) is composed of two parts--V.sub.(sig) and V.sub.(os). V.sub.(sig) is a function of the proximity of the gear tooth to the Hall plate and V.sub.(os) is an error component that is a function of various physical factors. When the measured output voltage signal from the two terminal pairs are combined, the error components largely cancel out. As a result, the effects from the practical problems of using Hall effect sensors are minimized.
The circuitry that processes the output voltage from the four terminal Hall device is shown in FIG. 2. This circuitry in combination with the Hall device of FIG. 1 creates a Hall sensor. In the circuitry of FIG. 2, the output terminals 15, 15' from the Hall device are connected to operational amplifier ("op amp") 40. The output of op amp 40 is stored in capacitor 48 when a clock signal from the clock 42 has a high logic state. The output from Op Amp 40 is stored in capacitor 46 when clock 42 has a low logic state. The switch pairs 12 and 14 also operate in conjunction with the clock 42. As a result, the output voltage measured between the terminal pair 4 and 8 is stored in capacitor 48 and the voltage measured at terminal pair 2 and 6 is stored in capacitor 46. Both capacitors 48 and 46 have a common connection which, in effect, combines the two output voltage signals. Finally, op amp 47 then creates the final Hall sensor output signal.
While the Hall sensor shown in FIGS. 1 and 2 does minimize some problems associated with general Hall sensors, it also creates problems of its own. In particular, the introduction of capacitors 46 and 48 to store the output voltage signal inherently introduces a timing delay in the system. That is, because of at least the capacitors 46 and 48, the time at which the output signal from amplifier 47 changes in response to the passing of a gear tooth may be significantly delayed from the actual time the gear tooth passed the Hall plate. This delay in the system impairs the performance of the Hall sensor in various applications.